I know code allows open risers but I am trying to find more definitive information on safe design. As usual, opinion and anecdote is easy to find but I haven't seen a more academic/scientific look at, for example, accidents and the presence/absence of risers (though of course, 'correlation does not imply causation').
edit: I found related information on pubmed (https://www.ncbi.nlm.nih.gov/pubmed/26203488) regarding highlighting the edge of the step to improve gait on the staircase. This concept could be applied to my question if the open riser provided contrast to the stair itself. Unknown whether the open riser itself causes issues (e.g., foot catching in the open space).